The Aryl Hydrocarbon Receptor (AhR) is a ligand -inducible transcription factor and a member of the basic helix-loop-helix/Per-Arnt-Sim (bHLH/PAS) family. While a high affinity endogenous ligand for the AhR has yet to be identified, the importance of the normal functioning of the AhR is evidenced by the evolutionary stability of this receptor in the tissues of all vertebrates and many invertebrates. Studies in AhR knockout mice confirm the vital role for this receptor in tissue growth and differentiation. Until recent years, most of the interest in the AhR was related to the key role of this signaling protein in the biological and toxicological effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) and other persistent halogenated aromatic hydrocarbons (HAHs). Several HAHs bind with high affinity to the AhR and induce or repress the transcription of a wide range of genes, which in turn leads to alterations in a wide array of biological systems. More recently, a number of studies have reported that a wide range of structurally diverse exogenous and endogenous chemicals can bind to and activate AhR cell signaling. These natural and synthetic chemicals are generally rather weak ligands that have structural and physiochemical characteristics strikingly different from the "classical" HAH ligands. Based on the rather promiscuous nature of the AhR binding site and the structural diversity of AhR ligands, our hypothesis is that peptides may serve as agonists or antagonists of AhR signal transduction. This hypothesis will be addressed through the following specific aims. 1) Utilize phage display libraries to identify peptides that have affinity for the AhR. 2) Determine the ability of candidate peptides to activate the AhR complex and initiate AhR-dependent gene expression utilizing: a) an enzyme immunoassay for the ligand activated AhR and b) a reporter gene bioassay. Data analysis will include BLAST searches with the peptides in an attempt to identify proteins that may serve as endogenous ligands and molecular modeling to compare the structures of the wide range of known AhR ligands to the peptide(s). Relatively small peptides or proteins may serve as endogenous ligands that regulate the cell-specific physiological roles of the AhR. Alternatively, even if it is not the endogenous ligand, the discovery of a functional peptide Iigand for the AhR may provide a useful tool to further investigate the biological and toxicological responses that are mediated through AhR activation. [unreadable] [unreadable] [unreadable] [unreadable] [unreadable] [unreadable]